In solid state illumination, when high brightness together with high flux, or when high brightness together with color tuning, are required, the only possibility is to use several high brightness LEDs (HBLEDs), or multi-chip LED packages. In general, the optics problem consists of transmitting the light emitted by the LEDs to a target with a prescribed irradiance pattern. When high brightness is desired in illumination, it is because of the need to achieve reasonably sized optical systems (maximum intensity of a system is upper bounded by the product of the highest brightness of the LEDs times the optical aperture area). In general, the luminaires' emission angle is small in high brightness applications, which implies that the optical system aperture size is relatively large. There are two basic strategies in designing a luminaire optical system for multi-LED systems:
Multiple Individual Optics (MIO): where every LED chip has its own individual optical system and all individual optical systems are identical,
Single Common Optics (SCO), where there is a single optic collecting and processing the light from all the LEDs. This single optics needs the light from the multiple sources to be from a single connected region, otherwise the optics will send to the target high brightness rays mixed with low brightness rays, which results in a system which “dilutes” the brightness (and so increases its size). To be more precise, the single optics needs the light being carried by a ray bundle, when represented in ray phase space, to form a connected region. A simple way to get a single connected LED source is just to pack all the LED chips close together, as in a multi-chip LED package.
There are several aspects differentiating both strategies:
Thermal management: Because LED chips are tightly packed in the SCO strategy; heat extraction is much more difficult in this case than in the MIO strategy.
Electrical interconnections: The tight package of the SCO strategy makes electrical interconnections and chip electrical isolation more difficult than in the MIO strategy, although this may be a relatively small problem for some soldering technologies.
Volume of the optical system: The volume of the optical system is proportional to the third power of diameter of the emitting area (roughly speaking). In the MIO strategy, the emitting area of each chip is comparatively small. Assume that the diameter of a single LED is d and there are M LEDs. The volume of each individual optics in the MIO strategy is proportional to d3 and so the total volume of optical parts grows as M d3. In the SCO strategy the diameter of the chip array is proportional to M1/2 d and so the volume of the optical system grows as (M1/2 d)3, i.e. M1/2 times faster than in the MIO strategy.
Optical system manufacturing and assembling: This is the weakest aspect of the MIO strategy because the emission angle in high brightness applications is usually small and so aiming the different optics towards the same direction becomes more difficult. This is not only because the number of optics to align becomes large, but also because the aiming angle error must be much smaller than the emission angle, which is already small.
Number of different optical parts. In general the number of different optical parts in the MIO strategy is much larger, unless all the different optical parts are manufactured in a fewer number of pieces, for instance injecting all the optics in a single piece. Nevertheless, injecting all the optics in a single piece may worsen the optical quality. This is because the corners around every individual optics cause warpage and uneven shrinkage leading to misalignment of the pieces as well as worsening the optical quality of the surfaces.
A hybrid strategy between SCO and MIO could reduce the number of chips per cluster but still have multiple optics (one per cluster). This strategy diminishes the pros of one option by increasing the pros of the other option.
The same idea presented here for LEDs is also applicable to Concentrating Photovoltaics (CPV). Most of the CPV systems use the MIO strategy, and a few of them (for instance Solar Systems) uses the SCO strategy (an array of solar cells as a receiver and a single optics for the whole array). The idea presented henceforth is also applicable to CPV.